Explosion characteristics of micron- and nano-size magnesium powders were determined using CSIR-CBRI 20-L Sphere, Hartmann apparatus and Godbert-Greenwald furnace to study influence of particle size reduction to nano-range on these. The explosion parameters investigated are: maximum explosion pressure (P-max), maximum rate of pressure-rise (dP/dt)(max), dust explosibility index (K-St), minimum explosible concentration (MEC), minimum ignition energy (MIE), minimum ignition temperature (MIT), limiting oxygen concentration (LOC) and effect of reduced oxygen level on explosion severity. Magnesium particle sizes are: 125, 74, 38, 22, 10 and 1 mu m; and 400, 200, 150, 100, 50 and 30 nm. Experimental results indicate significant increase in explosion severity (P-max: 7-14 bar, K-St: 98-510 bar.m/s) as particle size decreases from 125 to 1 mu m, it is maximum for 400 nm (P-max: 14.6 bar, K-St: 528 bar.m/s) and decreases with further decrease of particle size to nano-range 200-30 nm (P-max: 12.4-9.4 bar, K-St: 460 -262 bar.m/s) as it is affected by agglomeration of nano-particles. MEC decreases from 160 to 30 g/m(3) on decreasing particle size from 125 to 1 mu m, its value is 30 g/m(3) for 400 and 200 nm and 20 g/m(3) for further decrease in nano-range (150-30 nm). MIE reduces from 120 to 2 mJ on decreasing the particle size from 125 to 1 mu m, its value is 1 nil for 400, 200, 150 nm size and <1 mJ for 50 and 30 nm. Minimum ignition temperature is 600 degrees C for 125 mu m magnesium, it varies between 570 and 450 degrees C for sizes 38 -1 mu m and 400-350 degrees C for size range 400-30 nm. Magnesium powders in nano-range (30-200 nm) explode less violently than micron-range powder. However, likelihood of explosion increases significantly for nano-range magnesium. LOC is 5% for magnesium size range 125-38 mu m, 4% for 22-1 mu m, 3% for 400 nm, 4% for 200, 150 and 100 nm, and 5% for 50 and 30 nm. Reduction in oxygen levels to 9% results in decrease in P-max and K-St by a factor of 2-3 and 4-5, respectively, for micron as well as nanosizes. The experimental data presented will he useful for industries producing or handling similar size range micron- and nano-magnesium in order to evaluate explosibility of their magnesium powders and propose/design adequate safety measures. (C) 2013 Elsevier Ltd. All rights reserved.